Master Cylinder Bench Bleeding: Isn't it redundant?
 

I'm replacing my brake master cylinder later this week after 6 months of not having my E53 while waiting to get some parts to complete the new lift in our garage :-D

Every resource seems to suggest I need to Bench Bleed the master cylinder.

If I'm flushing and bleeding the entire system at the calipers, isn't that just redundant work? Or is there some specific reason that makes it important to bleed the MC before bleeding the rest of the system?

Thanks!

It is a good idea and doesn't hurt anything, but remember that you MUST use something like a Foxwell or 1.4 to trigger the ABS block or you will never get bubbles out of the system completely.

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Bench bleeding flushes out all the manufacturing grease and dirt, otherwise all that stuff has to flow through the entire brake system. Bench bleeding is nothing more than using a dowel or phillips screw driver to push the piston in while the mast cyl has fluid. With a few pumps your done and all "Primed up". Yes, you can buy the speciality tool lines to flow the line outlets back into the master cyl reservoir or just let it flow on to the bench shop towel.

Normal Brake Bleeding procedure
Install the master and reservoir, pressurize the brake system using a brake tool, bleed out the RR, turn on the Foxwell or other activation software tool, cycle the "ABS Brakes Solenoids" that will open up all the passageways in the ABS block, run the systems test and PRESS the brake pedal 5 times with the entire set up still attached. In other words, pressure on master cyl, fluid full in master cyl, caliper bleed screw open, catch can in place, ABS finished cycling, pump pedal 5X. More to LR, RF, RR. Repeat one more time until all fluid flow is without ANY bubbles.

Road test

Rebleed one last time using all the steps.

Done put away the tools and clean up.

Anal? Yes but it works every time.

Bench bleeding is to remove as much of the air that is in the master cylinder. A Master cylinder is shipped dry so you want to remove as much of the air that is in the piston area of the master cylinder before connecting it to the brake system. If you don't let the system bleed out you shouldn't need to bleed the ABS pump.

Do you use Foxwell to do this or ISTA/D? Curious where in ISTA/D you go to open the solenoids? I found a menu option to temporarily engage them but they didn't stay open. Are you speaking of the temporary engagement or did you get them to stay open while you were bleeding?